It may not be a big job, but it also may!
If you have inherited a historical problem, they maybe a reason why you have what you have. I would personally stump 100 Great British Pounds and hopefully the electrician is as good as his/her word!
If you are certain about the 3 core feeding the alarm panel, it must have sounded off by now as the panel will not have power the way the 3 core is connected now.
Alternatively, the 3 core could be feeding the second switched fitting: red=L blue=N yellow=SW L.
Sometimes I have taken a three core to fittings with a permanent live in case I need to extend in the future.
To advise further, you need to post pictures of the wiring in the other rose and the switches.
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